A tremendous amount of literature exists in zooarchaeology regarding
the effects of cooking on bone.  There is also a tremendous amount of
misunderstanding regarding what happens when bone is cooked.  For the
sake of simplicity, we can limit the discussion to two types of cooking:
wet and dry.  Dry cooking, such as roasting, baking, cooking over an open
flame, etc., has four major effects on bone.  First, it drives off water
attached to the bone mineral, changing its basic structure and generally
making it harder.  Second, it oxidizes carbon compounds within the bone
(the cartilaginous matrix), burning them away or weakening them
significantly, reducing flexibility.  Third, long-term heat shrinks bone,
resulting in longitudinal and horizontal cracking, exfoliation of the
outer cortical layers of compacta, and splitting.  Last, if the heat
is high enough or long enough, the hydroxyapatite can melt and
recrystallize, forming compounds with different physical properties.
In general, dry cooking results in bone that has reduced pliability,
fractures easier, and has sharper edges and points.  Bones that have
been subjected to dry cooking become quite hard, and will preserve for
millennia.  If you strike them with a metal pick, they will ping like
ceramics.
 
Wet cooking, such as pressure cooking, boiling, and stewing, is a
different bowl of soup.  First, because most meats and fats are slightly
acidic (about pH 4.5 to 6), boiled bones are exposed to corrosive
agents that cause pits, cracking, and exfoliation of the outer cortical
surfaces.  Interestingly, these dissolution changes are similar to
those caused by gastric acids in contact with bone surfaces, although
not as extreme.  Additionally, as hot water is forced into the bone, it
dissolves the minerals and cooks the proteins that bind the salts within
their matrix.  This causes a tremendous amount of bone deterioration.
Bones that have been boiled are lighter, chalkier, and unable to
withstand the same physical stresses as fresh or dry cooked bones.  If
boiled long enough, bones will decalcify to such an extent that they can
be smashed into a paste with fingertip pressure.  Anyone who has boiled
chicken knows how soft the bones become.  If you strike boiled bone with
a metal pick, it will thunk, not ping.
 
Cooking a bone does NOT change its reactivity to gastric acids; they
still dissolve regardless of cooking time and methods.  However, the fact
that wet-cooked bones have undergone dissolution changes makes gastric
digestion faster and more efficient compared to bones that have been
dry-cooked.  In terms of risk, it is probable that there is more chances
of problems from ferrets consuming dry-cooked bones compared to raw
bones, and more risk from eating raw bones than wet-cooked ones.  Even
so, the risk from eating ANY type of bone would be relatively small.
 
Bob C
[Posted in FML issue 4158]